1. Field of the Invention
The present invention relates to pigments and optical sheeting, particularly multilayer pigments and multilayer optical sheeting, more particularly multilayer pigments and sheets with optically variable properties.
2. Background of the Art
Pigments are materials, which are added to other materials to create a visual effect, usually the provision of color to an observer. As opposed to dyes, pigments are differentiated by their lack of solubility in the medium within which they may be carried. The same compounds may therefore be pigments in one medium and dyes in another. Pigments absorb radiation and allow non-absorbed radiation to be transmitted through them or to be reflected off the pigment to provide the appropriate color.
Pigments are substantial items of commerce, being used in millions of metric tons per year (especially if traditional white and black fillers such as titania, silica, zinc oxide, carbon black, etc. are considered as pigments). Pigments are used in paints, fabrics, polymer coatings, inks, toners, ceramics, articles of manufacture (e.g., watches, floppy disks, identification cards, credit cards, rubber items, etc.), decorative materials, and the like.
One type of pigment is alternatively referred to as a luster pigment, optically variable pigment, internal reflectance/refraction pigment, internal reflection/interference pigment, effect pigments, plate-like pigments, etc. These pigments differ in the mechanism by which they fundamentally create the color values when light interacts with them. Rather than directly absorbing limited ranges of the wavelengths of light which strike the pigments, luster pigments provide three types of interactions with visible radiation which results in alteration of the viewable light leaving the pigments. The three types of interactions with light include 1) mirror reflection, in which light is reflected off the surface of the pigment, 2) absorption, transmission and reflection of light by the bulk composition of the pigment (an internal effect), and 3) interference and reflection. The optical effects that operate in luster pigments are the same principal involved in the multicolor effects one sees in soap bubbles. The colors observed in a soap bubble are various merging interference colors which are strongly angle dependent hues. The same optical effects are viewed in thin films of inorganic materials such as silica and magnesium fluoride, which are quite suitable for the pigment industry, while titania has a high refractive index. In the art, low refractive index materials are often referred to as dielectric materials (irrespective of their electrical insulating or storage properties) because many dielectrics are non-absorbing and low refracting. The interference colors of these types of materials are highly angle dependent as explained by Snell's Law: EQU n.sub.0 sin .alpha.=n.sub.1 sin .beta.
wherein n.sub.0 is the refractive index of air and n.sub.1 is the refractive index of the substantive material. Snell's Law correlates the refractive angles and the refractive indices. FIG. 1 shows two layers of a single article through which light rays pass at a face angle and a grazing angle. In the case of materials with low indices of refraction, the bending of the rays with respect to the perpendicular would be slight and the difference between pathlengths of the two rays would be high. This results in interference conditions for face angle rays and for grazing angle rays which are completely different, producing strongly angle dependent colors.
Optically variable pigments have been produced by providing these physical effects into pigments. These pigments were first described in U.S. Pat. No. 3,438,796, and later described in U.S. Pat. Nos. 4,434,010; and 5,401,306 and EP Publications 395 410 and 708 154 and German Patent No. 195 25 503. Plate-like iron oxide pigments have been produced comprising an inner reflector plate-like Fe.sub.2 O.sub.3 seed, a low reflectance SiO.sub.2 layer, and a selective reflective Fe.sub.2 O.sub.3 exterior layer. Aluminum based optically variable pigments (Al/SiO.sub.2 /Fe.sub.2 O.sub.3) have also been provided as totally reflecting Aluminum cores, a low reflectance SiO2 layer, and a selective reflective Fe.sub.2 O.sub.3 exterior layer (e.g., 25 nm layer thickness). The apparent color provided by the face angle and the grazing angle will depend upon both the thickness of the SiO.sub.2 layer and the angle of viewing. The colors may vary from greenish gold (320-350 nm layer thickness) to red (380-400 nm layer thickness) to violet (410-420 nm layer thickness) to weak copper (430-440 nm layer thickness) in the face angle color with the (varying SiO.sub.2 layer thicknesses). Similarly, the grazing angle color will vary from reddish gray (320-350 nm layer thickness) to gold (380-400 nm layer thickness) to green (410-420 nm layer thickness) to weak red (430-440 nm layer thickness) with the thickness of the SiO.sub.2 layer. The literature provides two primary methods of synthesizing optically variable pigments, physical vapor deposition (PVD) and the chemical coating/deposition of layers onto particles or flakes as wet chemical vapor deposition (CVD).
Physical vapor deposition is performed by the vaporization of the material to be deposited as layers, with each layer of material being separately vaporized and then deposited on the substrate. Chemical vapor deposition similarly lays down each layer separately and has been performed by suspension of flakes in a carrier medium (e.g., alcohol), and the addition of a reducible silane (e.g., tetraethoxysilane and ammonia) is performed. The resulting hydrolysis product condenses and forms an SiO.sub.2 layer as a smooth film on the substrate. Alternatively the SiO.sub.2 layer is deposited from a fluidized bed reactor. Special precursors other than the tetraethoxysilane are desirable for this reaction. After deposition of the SiO.sub.2 layer, metal oxide or metal film coatings are applied. This is usually done in a fluidized bed reactor, where gaseous metal carbonyls are decomposed at elevated temperatures (e.g., about 200.degree. C.), forming smooth thin films on the SiO.sub.2 layer surfaces. Carbonyls such as aluminum, chromium, molybdenum and tungsten are commonly used in this process.
One example of a synthetic resin-coated pigment using chemical deposition means is U.S. Pat. No. 5,332,767 wherein a synthetic resin coated pigment such as aluminum having a siloxane coating covalently bonded to the surface of the pigment particles is formed by hydrolysis and condensation of the silicon-organic coated on the particles. Similarly, U.S. Pat. No. 5,261,955 describes colored metal flakes manufactured by coating metal oxide sols onto the flakes, and thereafter heating the flakes to impart visible colors.
Multiple layer coatings on substrates are also taught in U.S. Pat. No. 5,182,143 wherein a coating solution contains two or more dissimilar hydrolyzable and condensable organometallic compounds which differentially condense into different layers.
U.S. Pat. No. 5,500,313 makes general reference to slot coating as an alternative in the manufacture of holographic flake pigment. The materials are coated, imaged and then converted into particulates for subsequent inclusion in binders.
U.S. Pat. No. 5,641,544 describes a method for forming ultrathin coatings of liquid onto substrates and discloses a multiple slot die coating system in which multiple layers of liquids are simultaneously deposited onto a surface. Amongst the materials described as being coatable by the process (column 8, lines 3-32) are optically active coatings, reflective sheeting, functional coatings, and the like. The coating fluids described include solutions, solid-fluid dispersions, fluid mixtures and emulsions.
U.S. Pat. No. 4,445,458 describes a novel head for a slot die coater, and generally describes slot die coating processes forming metered beads.
Vapor deposition and chemical deposition processes are time consuming, energy intensive, require strict environmental control, and are expensive for the formation of essentially low technology staples such as pigments. Improved processes for the formation of optically variable pigments are desirable.